The body weight rats actively maintain is controlled by diet and dietary regimen. Does exercise similarly control defended body weight? Normal rats and rats with ventromedial hypothalamic lesions will run on a motorized treadmill for fixed distances and at a fixed rate daily. When food intakes and body weights have stabilized for any given level of exercise, intragastric feeding (to decrease caloric demands on voluntary intakes) or reduction of ambient temperatures (to increase caloric demands) will indicate whether these stable body weights are defended body weights or the passive consequence of a stable food intake determined by other mechanisms. Knowledge of situational factors that determine what body weights rats defend are of potential significance for the treatment or prevention of human obesity. It has become necessary to distinguish at least three definitions of the defense of body weight, and hence three definitions of the hunger for calories. Electrolytic lesions of the "lateral hypothalamic hunger center" have produced various behavioral syndromes analyzable according to these distinctions. Histological analysis must be performed and related to these syndromes. Histological analysis is also required for similar behavioral syndromes that have resulted from midbrain tegmentum lesions. The production and analysis of more discrete lesions in these two regions, and of lesions in the substantia nigra and globus pallidus (terminals of the nigrostriatal pathway) will help define further the hunger constructs needed for describing both normal behavior and the effects of these lesions, and the possible neuroanatomical bases for these distinctions. Also assessing temporary and permanent thirst deficits and the role of taste in certain of the behavioral syndromes will aid in conceptualizating the cause of aphagia and adipsia and of recovery in rats with these lesions. Ability to adequately conceptualize the defenses of body weight and th hungers of calories, and to mechanistically describe them, should provide clues for rational treatment of disorders of hunger and of body weight, and also should aid conceptualizing motivational constructs and systems generally.